Metallic articles are fabricated by any of a number of techniques, as may be appropriate for the nature of the metal and the article. In one common approach, metal-containing ores are refined to produce a metal. The metal may be further refined as necessary to remove or reduce the amounts of undesirable minor elements. The composition of the refined metal may also be modified by the addition of desirable alloying elements. These refining and alloying steps may be performed during the initial melting process or after solidification and remelting. After a metal of the desired composition is produced, it may be used in the as-cast form for some alloy compositions (i.e., cast alloys), or further worked to form the metal to the desired shape for other alloy compositions (i.e., wrought alloys). In either case, further processing such as heat treating, machining, surface coating, and the like may be employed.
One of the most demanding applications of materials in aircraft gas turbine engines is the disks (sometimes termed “rotors”) upon which the turbine blades or compressor blades are supported. The disks rotate at many thousands of revolutions per minute, in an elevated-temperature environment, when the gas turbine is operating. They must exhibit the required mechanical properties under these operating conditions.
Certain ones of the gas turbine engine components such as some of the disks are fabricated from titanium alloys. The disks are typically manufactured by furnishing the metallic constituents of the selected titanium alloy, melting the constituents, and casting an ingot of the titanium alloy. The cast ingot is then converted into a billet. The billet is further mechanically worked, typically by forging. The worked billet is thereafter upset forged, and then machined to produce the titanium-alloy component.
Small mechanical or chemical defects in the final disk may cause the disk to fail prematurely in service. Mechanical defects include, for example, cracks and voids. Chemical defects include, for example, hard alpha defects (sometimes termed low-density inclusions) and high-density inclusions. Hard alpha defects, discussed for example in U.S. Pat. Nos. 4,622,079 and 6,019,812, whose disclosures are incorporated by reference, are particularly troublesome in premium-quality alpha-beta and beta titanium alloys used in demanding gas turbine engine applications, as well as other demanding applications such as aircraft structures. Chemical defects may cause cracks to form prematurely in engine service. A failure resulting from these defects may be catastrophic to the gas turbine engine and possibly to the aircraft. Consequently, it is necessary to fabricate the gas turbine engine disk with great care to minimize and desirably eliminate the presence of such defects, and to produce the disk in a manner that facilitates its ultrasonic inspection to detect such defects if they are present. The manufacturing process must also produce a microstructure in the final article that exhibits the desired combination of mechanical properties and physical properties required in the disk.
It has been possible, using existing melting, casting, and conversion practice, to reduce the presence and size of chemical defects in installed disks to reasonably low levels. However, there is always a desire and need for a manufacturing process to produce the disks and other components with a further reduction in the incidence of such chemical defects, thereby improving the operating margins of safety. The present invention fulfills this need for an improved process, and further provides related advantages.